The purpose of this study is to evaluate how safe and how effective bosentan is in combination with sildenafil. The combination use will be compared to subjects receiving sildenafil only. Potential subjects will be patients using sildenafil for the treatment of pulmonary arterial hypertension (PAH) who still have symptoms.

The purpose of this study, which is also called the main study, is to evaluate the safety and effectiveness of ambrisentan compared to placebo (a tablet that looks like ambrisentan but contains no medicine) in subjects with PH associated with IPF.